The RELIEF flow tagging technique and its application in engine testing facilities and for helium-air mixing studies

Raman excitation plus laser-induced electronic fluorescence (RELIEF) images the motion of oxygen molecules in air and other gas mixtures. This is accomplished by tagging oxygen molecules through vibrational excitation and imaging them after a short period of time by laser-induced electronic fluorescence. The vibrational lifetime of oxygen is sufficiently long and the signal sufficiently strong to allow this technique to be used over a wide range of flow conditions, from low subsonic to hypersonic, and in a variety of gas mixtures including high humidity environments. The utilization of a molecular tagging technique such as this is critical for environments in which seeding is impossible or unreliable and for measurements in which a wide range of scales needs to be observed simultaneously. Two experiments which have been conducted at national laboratories in medium- to large-scale facilities are reported. At the Arnold Engineering Development Center, RELIEF was used to examine velocity in a 1 m diameter tunnel for applications in the area of engine testing. At the NASA Langley Research Center, RELIEF is being used to examine supersonic mixing of helium in air in a coaxial jet in association with studies of fuel-air mixing in hypersonic engines. These applications are two examples of the wide range of practical uses for this new technology.